This proposal outlines a program to develop a new basis for assessing the nature of the tautomeric equilibria of biologically important purine and pyrimidine bases and their nucleosides and, in addition, to further develop the structural and analytical utility of magnetic circular dichroism spectroscopy. It is planned to measure the magnetic circular dichroism spectra of important purine and pyrimidine bases and to compare the data obtained in solution with the corresponding data obtained under the same conditions on model compounds made by the replacement of hydrogens by alkyl groups which block the tautomeric transformation. The nature of the tautomeric equilibria will be detected by comparing the MCD spectra of the base with the spectra of the methylated analogs representing all possible tautomeric forms or the most likely tautomeric forms. A second series of studies will be concerned with the different factors capable of influencing the tautomeric equilibria such as solvent, substituent, acidic or basic character of the solvent, etc. Projects are described which cover virtually every aspect of this actively developing field of research including the prototropic, the keto-enol, the amine-imine and the thione-thiol tautomerism of the major and minor nucleic acid bases in relation to their physico-chemical and biological properties.